President George Bush has told US troops to be ready for 'pre-emptive military action' against Iraq, as security sources warned that a massive assault against President Saddam Hussein could be likely at 'short notice'.

Whitehall sources confirmed that Tony Blair had decided Britain must back any US assault and had ordered defence planners to begin the preparations for a new war in the Gulf.

'President Bush has already made up his mind. This is going to happen. It is a given,' said one Whitehall source. 'What we are waiting for is to be told the details of how and when and where.'

Although Britain has not decided on its level of commitment, defence sources say planners have been told to expect to send 20,000-30,000 British troops.

The sources added that British Challenger II main battle tanks and other key armoured fighting vehicles were being pushed through a crash servicing and refit programme. The Ministry of Defence has explained the crash repairs programme by saying it is for a military exercise planned for Scotland.

However, expectation of a large British involvement in a US-led war to topple Saddam Hussein has been raised by reports that Britain will issue an emergency call-up of reservists in September and by reports of other preparations, including a big increase in RAF training flights.

'The combat indicators are all there,' said one source. 'This is going to happen. And perhaps sooner than we think.'

Whitehall sources claim, however, that the Prime Minister is hesitating in declaring his full endorsement of Bush's plans until Washington puts in a formal request for British troops.

Unlike Bush, Blair is understood to be concerned that Britain can make a legal case for intervening in Iraq to remove Saddam, because of concern that his support for the war could split the Cabinet and lose the support of the Parliamentary Labour Party.

Blair ordered the preparation of a document that would lay out the justification for attacking Iraq three months ago. Sources say the document - expected to set out a 'legal framework' for a war - has been completed.

The latest disclosures came as Bush used a visit to the troops that fought al-Qaeda forces in Afghanistan to renew his vow that the United States would strike pre-emptively against countries developing weapons of mass destruction, telling troops that 'America must act against these terrible threats before they're fully formed'.

Surrounded by troops of the 10th Mountain Division, among the first sent to Uzbekistan and Afghanistan, one of the soldiers yelled: 'Let's get Saddam!'

Bush's address came amid reports of efforts by Iraqi diplomats to court Arab neighbours in countries that might be used for a US assault.

Iraq began to end a decade of diplomatic isolation in March at the Arab summit. Since then - according to the Washington Post - it has signed up to economic agreements with Egypt, Saudi Arabia and Turkey, and discussed prisoner exchanges with Iran, putting pressure on Washington to act swiftly.